<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Starry Sky</title>
    <style>
        body {
            margin: 0;
            overflow: hidden;
            background-color: #000;
        }

        .canvas {
            position: fixed;
            top: 0;
            left: 0;
            z-index: -1;
            filter: alpha(opacity=20);
        }
    </style>
</head>
<body>
    <canvas id="canvas" class="canvas"></canvas>
    <script>
        function drawStars() {
            var canvas = document.getElementById('canvas'),
                ctx = canvas.getContext('2d'),
                w = canvas.width = window.innerWidth,
                h = canvas.height = window.innerHeight,

                hue = 255, // 色调色彩
                stars = [], // 保存所有星星
                count = 0, // 用于计算星星
                maxStars = 2000; // 星星数量

            var Star = function() {
                this.orbitRadius = random(maxOrbit(w, h));
                this.radius = random(60, this.orbitRadius) / 8;
                this.orbitX = w / 2;
                this.orbitY = h / 2;
                this.timePassed = random(0, maxStars);
                this.speed = random(this.orbitRadius) / 500000;
                this.alpha = random(2, 10) / 10;

                count++;
                stars[count] = this;
            }

            Star.prototype.draw = function() {
                var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
                    y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
                    twinkle = random(10);

                if (twinkle === 1 && this.alpha > 0) {
                    this.alpha -= 0.05;
                } else if (twinkle === 2 && this.alpha < 1) {
                    this.alpha += 0.05;
                }

                ctx.globalAlpha = this.alpha;
                ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
                this.timePassed += this.speed;
            }

            function random(min, max) {
                if (arguments.length < 2) {
                    max = min;
                    min = 0;
                }

                if (min > max) {
                    var hold = max;
                    max = min;
                    min = hold;
                }

                return Math.floor(Math.random() * (max - min + 1)) + min;
            }

            function maxOrbit(x, y) {
                var max = Math.max(x, y),
                    diameter = Math.round(Math.sqrt(max * max + max * max));
                return diameter / 2;
            }

            var canvas2 = document.createElement('canvas'),
                ctx2 = canvas2.getContext('2d');
            canvas2.width = 100;
            canvas2.height = 100;
            var half = canvas2.width / 2,
                gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
            gradient2.addColorStop(0.025, '#CCC');
            gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
            gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
            gradient2.addColorStop(1, 'transparent');

            ctx2.fillStyle = gradient2;
            ctx2.beginPath();
            ctx2.arc(half, half, half, 0, Math.PI * 2);
            ctx2.fill();

            for (var i = 0; i < maxStars; i++) {
                new Star();
            }

            function animation() {
                ctx.globalCompositeOperation = 'source-over';
                ctx.globalAlpha = 0.5;
                ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 2)';
                ctx.fillRect(0, 0, w, h)

                ctx.globalCompositeOperation = 'lighter';
                for (var i = 1, l = stars.length; i < l; i++) {
                    stars[i].draw();
                };

                window.requestAnimationFrame(animation);
            }

            animation();
        }

        drawStars();
    </script>
</body>
</html>
